Fuses that are suited for use in automobiles and other circuit protection purposes may be found in both male- and female-type configurations. Many such fuses are two-piece assemblies.
One common configuration includes a box-like housing and an all metal male or female one-piece fuse element secured within that box-like housing. Some such prior female fuse assemblies have a metal female fuse element with a pair of spaced-apart female terminals which are accessible from one end of the housing. The female terminals are closely proximate to the housing walls.
An unsupported metallic fuse link is typically suspended between the extensions of the female terminals. The metallic fuse link is closely spaced from the housing side walls. A low fusing point metal is typically attached to the metallic fuse link.
The housing has slot-like openings at one of its ends, and the female terminals are accessible from these slot-like openings. Particularly, male blade-type terminals can be inserted through these slot-like openings to access the female terminals. These male blade-type conductors typically extend from a mounting panel or fuse block. Typical one-piece female fuse elements and the methods of making them are described in U.S. Pat. Nos. 4,344,060, 4,570,147, 4,751,490 and 4,958,426.
Automobile and other female fuse assemblies also have included an all metal female three-piece fuse element in place of a one-piece fuse element. As in the previously mentioned female fuses, the metal female fuse element has a pair of spaced-apart female terminals which are accessible from one end of the housing. The female terminals can be created from typical male terminals by adding female sockets to the male terminals, rather than forming the complete female fuse element from one piece. This structure and method of making such a fuse is described in U.S. Pat. Nos. 4,672,352 and 4,869,972.
There are several constraints which exist when working with a one-piece female fuse construction. For example, the stiffness or resilience (spring qualities), as well as the conductivity, of the fuse element material become important factors in determining the materials to be used. It is clear that the conductivity of the material is important, because of the principle that unnecessary resistance will increase the voltage drop of the fuse, thus reducing the amount of current flowing through the fuse. The resilience of the material is also important because the female engagement portion of the female fuse element must be durable and spring-like in order to continuously grip the male terminals on the terminal block in a snug manner. The resiliency is important in view of gravitational forces exerted on the fuse element when current heats up the fuse element, as described in U.S. Pat. No. 4,635,023.
When determining an appropriate construction for a three-piece fuse, the designer can choose materials for the fuse element which are different from the materials of the female sockets. Specifically, the designer may choose a material for the fuse element which will allow for suitable conductivity, while at the same time the designer can choose a different material for the female sockets which will provide ample resilience to effect a snug fit between the fuse element, sockets, and male terminals inserted in the female socket. A snug fit will keep the resistance, and thus the current loss, low between the terminals of the fuse element and male terminals connected or linked thereto by the sockets.
A snug fit only exists if there is practically no movement between the fuse element, sockets, and male terminals inserted in the sockets. These elements should also remain still, relative to their housing, to prevent the snug fit from being broken by any movement between these elements. If the fit between the fuse element, sockets and male terminals does not remain snug over time, the resistance will increase and become unsatisfactory for prolonged commercial use.
Although U.S. Pat. No. 4,869,972 to Hatagishi discloses a three-piece female fuse configuration, this patent does not disclose a configuration that lends itself to a prolonged snug fit. The female sockets from this patent are disclosed as being used for testing. It is believed, however, that if this configuration was placed in a commercial environment (i.e., onto a male fuse block within an automobile), small vibrations in the commercial environment would cause the fit between the fuse element, sockets and male terminals to move about and loosen. Without a snug fit, movement between these elements would cause a higher resistance within the fit, causing a loss of current as well as unwanted heating of the fuse connections near the fuse block.
U.S. Pat. No. 4,672,352 also discloses a three-piece fuse assembly which includes a fuse element, tab insertion sockets, and a housing to house the element and sockets. The focus of this patent is that the fuse element can be replaced without replacing the sockets or housing. Thus, construction of the housing allows for the fuse element to be removed without removing the sockets. This construction also appears to fail to provide firm fit of the sockets or fuse element within the housing, unless a male terminal is inserted in the sockets to force these elements outward from the male terminal. In addition, the fuse element is not secured to the socket in any way. The sockets are secured to the housing in a manner independent to the securement of the fuse element to the housing. If the fuse terminal moves within the housing, not only will the fuse element move in relation to the housing, but it will also move in relation to the sockets. Movement of the fuse element would also likely take place relative to the male terminal.
Resettable fuses that include a polymeric PTC material in lieu of a conventional, metallic fusible link are now available on the market. They are sold in various different configurations, none of which is like those of the present invention. Some of these prior art resettable circuit protection devices include a PTC element having a plate-like appearance and comprising a thin layer of a PTC material having a pair of thin coatings of metal forming terminals or electrodes on the opposite faces of the PTC layer. A pair of thin wire leads are electrically secured by solder to the opposite conductive faces of the PTC layer.
A variety of PTC elements like that just described are referred to as resettable fuses and are sold under the registered trademark POLYSWITCH.RTM. by the Raychem Corporation of Menlo Park, Calif. The maximum continuous, non-hazardous current of these POLYSWITCH.RTM. fuses that will not cause the PTC element at 20.degree. C. to switch from its low to its high resistance state, referred to as the "holding current," presently spans the current range of about 0.9-9.0 amps. The range of trip currents, which is the minimum continuous current that will cause the POLYSWITCH.RTM. fuses to be switched or tripped to a high resistance circuit-protecting state at 20.degree. C., varies from about 1.8 to 18 amps. This high resistance circuit-protecting state is maintained by a small, self-heating trickle current. The largest fault current which such devices can interrupt without being damaged varies from about 50-100 amps. The initial minimum resistance of these circuit protection devices varies from about 0.02 to 0.20 ohms.
Polymer PTC materials which are believed to be used in such devices are disclosed in U.S. Pat. Nos. 4,237,441 and 4,545,926. These types of PTC materials generally include a mixture of organic crystalline polymers in which are distributed conductive particles which may include carbon black. In such materials, as current flow therethrough progressively increases, the materials are progressively heated until the current reaches the trip current level. At this level, the resistance of the material suddenly increases to a substantially higher level due to the volume expansion of the material. This expansion separates the conductive particles by larger distances, providing a greatly increased resistance to current flow between the particles.
Other generally relevant U.S. patents which disclose resettable fuses that include a polymeric PTC material in lieu of a conventional, metallic fusible link include U.S. Pat. Nos. 4,331,861, issued to Meixner on May 25, 1982; U.S. Pat. No. 4,698,614, issued to Welch et al. on Oct. 6, 1987; U.S. Pat. No. 5,142,265, issued to Motoyoshi et al. on Aug. 25, 1992; U.S. Pat. No. 5,153,555, issued to Enomoto et al. on Oct. 6, 1992; and U.S. Pat. No. 5,233,326, issued to Motoyoshi on Aug. 3, 1993.
The configurations in which PTC resettable fuse devices have been commercially available are not suitable for automotive fuse applications. Thus, the present invention provides for a resettable, automotive circuit protection device which includes female terminals and one or more PTC elements.